A magneto-optical recording and reproducing apparatus records data into a magneto-optical disk by firstly irradiating a light beam on a thin film of a magnetic material provided on a substrate to raise the temperature and change the magnetization direction in the irradiated area where the temperature thereof has risen, and reproduces the data therefrom by exploiting the fact that the deflecting direction of the light beam turns depending on the magnetization direction of the magnetic material when the light beam is reflected by the magnetic material.
The magneto-optical disk, which uses a light beam to record or reproduce the data, can record or erase the magnetic data in and from a microscopic area about the size of the wavelength of light. Thus, the magneto-optical disk has been put into practical use as a high-density recording medium with a large capacity.
Conventionally, the data have been recorded or reproduced using a record mark substantially the same as the spot diameter of the light beam. However, various methods using a record mark smaller than the spot diameter of the light beam have been proposed to increase the data recording density of the magneto-optical disk.
For example, Japanese Laid-open Patent Application No. 5-81717/1993 entitled "Magneto-optical Recording Medium and Recording Method thereof" discloses a magnetic super resolution disk and a recording method thereof.
According to the above disclosure, a light beam is irradiated to a magneto-optical recording medium including a recording layer and an in-plane magnetized reproducing layer from the reproducing layer side to raise the temperature of the reproducing layer in the irradiated area. Under these conditions, the temperature rises above a predetermined level in a portion (aperture) within the irradiated area, and the in-plane magnetization of such a portion changes to the perpendicular magnetization as the magnetic property of a corresponding portion of the recording layer is transferred, thereby making it possible to reproduce a record mark smaller than the spot diameter of the light beam.
However, although the light beam is generated by a constant driving current, the reproducing power thereof varies in response to the change of environmental temperature at the time of reproduction. When the reproducing power is insufficient, not only the aperture becomes smaller than the record mark, but also the output of a reproduction signal (hereinafter referred to as the main signal) from an intended track becomes weak. As a result, the main signal contains more noise signals, and the probabilities of readout errors increase.
To eliminate this problem, Japanese Laid-open Patent Application No. 5-144106/1993 discloses a magneto-optical disk and a reproducing method thereof, in which two types of areas are secured. More specifically, the to-be-reproduced data are recorded in one type of the area, while in the other type of the area, which is secured to control the reproducing power, the record marks and non-mark portions of the same length are recorded alternately, so that each non-mark portion is sandwiched by the recording marks. The data can be reproduced reliably by setting the light beam's reproducing power to a level such that an amplitude of a signal of the above recording mark pattern achieves its maximum.
In case of a magneto-optical disk having recorded a sector mark, the sector mark and a VFO (variable frequency oscillator) signal are reproduced; the VFO signal is a signal recorded after the sector mark to correct the data readout timing in response to the change of the rotation of the magneto-optical disk. Then, the ratio at which the amplitude of the VFO signal achieves its maximum is determined in advance using the amplitude of a signal of the sector mark as the reference. Accordingly, the sector mark and VFO signal are controlled to keep such a predetermined ratio at the time of reproduction.
However, when the amplitude of the reproduction signal in the main signal achieves its maximum, the light beam's reproducing power becomes so strong that the aperture is undesirably enlarged and the record marks in the adjacent tracks are also reproduced through the aperture, thereby increasing the output signal from the adjacent tracks (hereinafter referred to as the mixing signal). Thus, the resulting reproduced data include considerable noise signals and the probabilities of readout errors increase, which makes the reproduced data output unreliable. That is to say, reproducing power that makes the maximum reproducing signal amplitude does not necessarily make the least mixing signals, and hence setting the reproducing power at a level such that yields the maximum amplitude of the reproducing signal causes the reproduced data to include many mixing signal components, thereby increasing the probabilities of reproduction errors and making the reproduced data unreliable.
To obtain a reliable reproduction output with fewer readout errors, it is necessary to control the light beam's reproducing power in such a manner that a sufficient output of the main signal for the data reproduction is obtained, and the aperture is kept in a size such that minimizes the mixing signals.